This post is a much a note to myself for the next time this happens, sorry for the indulgence but I hope it helps someone else too. I've just lost too much time looking for the answer to this when it happens. As it's VERY important to get these steps exactly right, you just don't want to go with the "I think this was what I did last time" attitude.
Immediately after an automatic update from Microsoft I was no longer able to use my mouse within a Word 2007 document. It didn't matter if it was a new document or an existing one. The only thing I could make the mouse action on were the ribbons. Word allowed me to move around and work in the document with keyboard stroke but nothing was happening with the mouse.